Fuel stabilization

ABSTRACT

Distillate fuel is stabilized against degradation during storage by inserting into storage tanks solid materials containing polar sites to enable polar condensation of the fuel constituents active in degradation. Polyether or polyester polyurethane open cell foams are the prime solid stabilizers proposed.

This invention relates to a method and means of preventing degradationin liquid hydrocarbon fuels.

Some liquid fuels particularly distillates are stored in static storagesfor periods of up to several years.

Over extended periods of time, small but significant changes in fuelproperties can occur. Important properties which change with time arecolour and insolubles content. Insoluble materials can plug fuel systemfilters, reduce or alter the fuel flow through engine nozzles and formsludges in the fuel tanks. Formation of insolubles is an indication ofchemical reactions in a fuel. Chemical degradation of fuels is animportant aspect of fuel stability.

Storage stability of distillate fuels has been of modest concern forfuels made by refining processes based on straight run distillation.However, increasing quantities of heavy crudes and distillationfractions are being run in refineries using cracking processes toincrease the yield of middle distillate fuels. The cracked products,which contain chemically unstable species, are blended into straight runstreams. The unstable components, although diluted by the blending,still exert a strong influence on insoluble material formation,particularly for long storage periods.

Stabilizing additives have been proposed to reduce the extent ofinsoluble material formation in middle distillate fuels. Specificadditives, such as phenylenediamines and hindered phenols, have beenused. These additives have been found to be effective in preventing theformation of gums in gasolines and peroxides in jet fuels. They havebeen shown to be ineffective in preventing the formation of particulatematter in distillate fuels.

Additive manufacturers' experience with diesel fuels and home heatingoils has led them to recommend alkyl amines as stability additives.These effective additives contained alkyl amines, some in conjunctionwith a metal deactivator (MDA). These observations were made on severalstraight run distillates as well as 30% blend of catalytically crackedstock (light cycle oil--LCO) in a stable straight run fuel.

U.S. Pat. No. 3,701,641 proposes to stabilise distillate fuels againstdegradation using a polyamine having 2 to 6 amino groups and 25 to 50carbon atoms.

Cyclohexylamines have been proposed as distillate fuel stabilizers inU.S. Pat. Nos. 3,640,692, 3,336,124 and 4,040,799 and in French Patent1441717.

It is an object of this invention to provide a simple alternative meansto stabilizing fuels.

To this end the present invention provides a method of stabilizingliquid hydrocarbon fuels which comprises storing the fuel with solidscapable of removing from fuel those active elements which catalyse orparticipate in the degradation reactions.

These insoluble solid additives reduce the amount of fuel degradationmaterial which is suspended in the fuel or deposited on the walls of thecontainer. The solid additives useful in this invention are thought tocontain polar sites to enable polar condensation of the activeconstituents to occur. A solid additive with this capability acts as apreferred deposition site for molecules of insoluble material, and forprecursors of insoluble material in the fuel. These precursors arepartitioned between the solid additive and the fuel molecules. Reductionof the concentration of these precursors in the fuel reduces degradationof the fuel.

Application of solid additives to the interior of fuel system componentsfor fuel stabilization can be in the form of cellular foams, sponges,mesh, woven fabric, naturally entwined or bound fibre bundles, surfacecoatings, strips, films, or solids such as powders or particulatematerial encapsulated in fuel permeable containers. Preferred foams areopen cell polyurethane foams or polymeric foams including urea,carbamate, ester or amide groups available as polar sites.

Polyurethane (PU) foam is a solid which possesses high fuelstabilization performance when immersed in fuel during storage. Thesepolymers comprise large numbers of well defined polar functional groupswhich have affinity for unstable components in the fuel. The partitionof unstable precursors between solid and fuel strongly favours theirdepletion from the fuel to the foam.

Fuel degradation material is also deposited on the foam surface, ratherthan forming as suspended particulate matter in the fuel.

When polyurethane foams are employed it is generally preferred to usepolyether based polyurethanes because of their greater stability inavoiding degradation by hydrolysis but in some cases polyester basedpolyurethanes will be quite suitable due to their better stability infuels containing higher levels of aromatic hydrocarbons.

The method of this invention can be carried out either as a pre-storagetreatment or by inserting the foams into storage tanks. Generally thefoams are present in a concentration of at least 0.01% weight by volumewith a preferred concentration of 1 to 5 g. per litre of fuel.

By passing the fuel through a bed of solids as taught by this inventionor through an open cell foam, the stability of the fuel is improved andits storage life extended. This is particularly useful with distillateswhich are highly susceptible to degradation.

Generally however it is preferred to incorporate the solids or foamsinto fuel storage tanks. The foams are preferably open cell in structureand have a porosity which ensures that the volume of fuel displaced islow but still provides a large surface area for fuel contact.

A number of comparison tests of accelerated ageing of distillate fuelshave been carried out to quantify the fuel stabilizing effect of a widerange of PU foam samples.

The test results are illustrated in FIGS. 1 to 5 which in graph form,provide comparisons between fuel ageing in the presence of a range offoams listed in table 1 and compared in FIGS. 1 to 3 with reference fuelsamples while FIGS. 4 and 5 illustrate the effect of PU foams on fuelscontaining fuel deposit promoters.

                  TABLE 1                                                         ______________________________________                                        PU Foam    Cell   Polyol           Air Flow                                   No*        Type   Type       Code  (m.sup.3 /h)                               ______________________________________                                         #1        .sup. C.sup.a                                                                        Ether      PE850 .sup. N.D..sup.c                            #2        .sup. R.sup.b                                                                        Ester      ME020 N.D                                         #3        R      Ester      ME010 38.1                                        #4        R      Ester      ME015 40.9                                        #5        C      Ester      EF430  0.5                                        #6        C      Ether      PE900  2.5                                        #7        C      Ether      HR940  8.0                                        #8        R      Ester      ME020 N.D.                                        #9        R      Ester      ME030 26.4                                       #10        R      Ester      SFI   N.D.                                       #11        R      Ester      SFII  N.D.                                       #12        R      Ester      SFIII N.D                                        #13        R      Ether      SFIV  N.D.                                       ______________________________________                                         Classification of PU Foams used in Fuel Stabilization Experiments.            .sup.a C = closedcell foam structure                                          .sup.b R = reticulated (opencell) structure                                   .sup.c N.D. = not determined                                                  *Foams #1-9 were manufactured by Cable Makers Australia                       Foams #10-13 were manufactured by Scotfoam, U.S.A.                       

The polyester foams 2, 3, 4, 8 & 9 contain modified poly(diethyleneglycol)adipate with a density of 28 kg/m³ and a cell count of 5 to 30pores perlinear centimetre.

The polyether foams 1,5,6 & 7 contain poly(oxypropyl)

poly(oxyethyl) glycerol and have a density of 27 kg/m³ and 15 to 25pores per linear centimetre.

Foam 13 contains an acrylonitrile-styrene modified polyoxyalkylenepolyether resins.

A range of polyether and polyester PU foams were tested as stabilizingadditives for (light cycle oil/straight run distillate) fuel blendsduring storage for various time intervals, in the temperature range43°-120° C. Intervals of storage at ambient temperature, approximatelyequivalent to specific experimental accelerated fuel stressingconditions used in this work, were calculated to be as follows:

    ______________________________________                                        Fuel Stress  Ambient Temperature                                              (Temp - Time)                                                                              Equivalent (Years)                                               ______________________________________                                        43° C. -                                                                       6 months   2.0                                                        65° C. -                                                                      43 days     2.3                                                        80° C. -                                                                      13 days     1.9                                                        80° C. -                                                                      14 days     2.0                                                        120° C. -                                                                     72 hours    6.9                                                        120° C. -                                                                     96 hours    9.2                                                        ______________________________________                                    

FIGS. 1, 2, 4 and 5 show the amounts of total insolubles formed from thesame fuel blend during fuel ageing for five different time intervals atthree different temperatures.

In all fuel stressing trials, duplicate samples of the respective fuelblends without additives were carried through all procedures, forcomparison with duplicate sample blends containing additives. In someexperiments, known fuel destabilizing agents (deposit promoters) wereadded in conjunction with PU foam additives in order to determine theefficiency of PU foam in counteracting the effects of these agents onfuel stability.

The degree of degradation of fuel blends under the various experimentalconditions was determined by measurements of parameters considered to berelevant to fuel instability :

    ______________________________________                                         (i)  Particulate matter                                                                                 = Total Insolubles (mq/1)                          (ii) Adherent Gum                                                             (iii)                                                                              Filtration Index                                                         (iv) Colour (ASTM D1500)                                                      ______________________________________                                    

Where applicable, changes in mass of the PU foam additives after ageingwere also measured. Soluble gum concentrations, by ASTM D381, have beenshown to have only a tenuous link 10 with quantitative measurements offuel instability.

The Filtration Index [(iii) above] is the ratio by which the filtrationtime (sec) of an aged fuel, with or without additive, exceeded thefiltration time (sec) of the prefiltered, unaged fuel blend, under astandardized method of pressure filtration (29 kPa; 4.2 p.s.i.) throughWhatman No.540 cellulose fibre membranes with nominal pore size of 8.0micron. For the filtered, unaged reference fuel, replicatedeterminations showed good reproducibility (±5%), with the averagefiltration time being 90 sec. For the aged fuels, however,reproducibility of filtratation times for some duplicate samples was notas good as for the unaged fuel. Factors such as formation ofvariable-sized particulate matter during ageing of duplicate samples,and non-uniformity in peformance of the nominal pore size filtermembranes with filtration of contaminated liquids, may have contributedto variations, sometimes as much as ±20%, for aged fuel duplicates. Inmost cases, however, variations were <±10%.

Filtration times for aged-fuel duplicates were averaged and convertedinto the above Filtration Index for semi-quantitative filterabilityevaluation. Aged fuels with Filtration Index between 1.0 and 1.3 wereconsidered to have "good" filtration characteristics, between 1.3 and2.0 "fair", and greater than 2.0 were considered "poor".

Mild accelerated ageing of fuels at 43° C. is widely accepted as beingthe most realistic and accurate test for estimation of the long termstability of diesel distillates during bulk storage. The data in FIG. 1,therefore, are considered to be representative of the performance of PUfoam fuel stabilizing additives under field conditions Ageing at 43° C.generally requires relatively long trial periods (at least 13 weeks) forsignificant changes in the fuel to occur. The data in the FIG. 1 wasobtained after 26 weeks fuel ageing at this temperature, a periodequivalent to about two years ambient storage.

The effectiveness of the PU foams for fuel stabilization is clearlydemonstrated in FIG. 1. Total insolubles and particulate levels werevery low, close to experimental detection limits for those parameters,indicating very high fuel stability compared to the aged reference fuelPolyether foam #1 underwent slight physical disintegration duringprotracted fuel immersion, with visible foam cell fragments contributingto the particulate components, thus increasing total insolubles. Thisfoam, however, was the only foam in this trial which was not designedspecifically for fuel immersion. The latter group showed unaffectedstructural integrity.

Filtration Index values (FIG. 1) for all fuels aged with PU foamsindicated "good" filterability, equivalent to, or approaching that forthe clean, unaged reference fuel (1.0). The aged reference fuel had asignificantly higher Filtration Index of 1.6.

Improvement in colour of aged fuel blends in presence of PU foams wasgreatest for ageing periods less than the equivalent of 6 months atambient temperature.

Colour differences between fuel containing PU foams and aged referencefuel decreased as the period of ageing increased.

Greater colour stability of aged fuel was achieved when the foam/fuelweight/volume ratio was of the order of 5 g per litre. Foams #9, #14,#15 and #16 (Table 2) were effective in suppressing colour degradationcompared to the reference fuel after ageing for periods of up to 48hours at 120° C. (equivalent of 4.6 years storage at ambienttemperature). The ageing period is shown in hours in Table 2. The fuelsaged with foam, which showed improved colour stability, also had greatlyreduced formation of insolubles and improved filterability as seen inTable 2.

                                      TABLE 2                                     __________________________________________________________________________                               Insolubles                                                                          Filtration                                   Foam                                                                              Foam Foam                                                                              Colour                                                                             Colour                                                                            Colour                                                                             mg/L  Index                                        No. Type g/L 15 hrs                                                                             39 hrs                                                                            48 hrs                                                                             48 hrs                                                                              48 hrs                                       __________________________________________________________________________        NONE --  4.0  5.0 5.0  21    4.8                                           #9 ME030                                                                              3.8 3.0  3.5 3.5  2     1.1                                          #14 SFV  4.8 2.0  2.0 2.5  3     1.0                                          #15 SFVI 5.6 2.5  2.5 2.5  2     1.2                                          #16 SFVII                                                                              5.1 2.5  2.5 2.5  2     1.1                                          __________________________________________________________________________     Foam #9 was manufactured by Cable Makers Australia, and Foams #14, #15 an     #16 by Scottfoam, USA.                                                   

Colour was determined by ASTM DE1500. The colour of the unaged fuel was2.0.

Very low levels of total insolubles, combined with good filtrationcharacteristics and colour stability are properties not normallyassociated with aged distillates which contain 30% unhydrotreated LCO.The above data, therefore, demonstrate that fuel-immersible PU foamsexert a strong stabilizing influence on distillate fuel.

Under the experimental fuel ageing conditions shown in FIGS. 2 and 3,each of the PU foam samples selected for evaluation imparted significantimprovement to fuel stability. The level of total insolubles was reducedin almost all cases by more than 50%, generally by more than 70%, and insome cases by greater than 80%.

FIGS. 2 and 3 also show no apparent discrimination between polyether orpolyester PU foams with respect to their fuel stabilizing properties atthe experimental fuel foam ratios.

The physical form of the foams possibly exerted a small influence onfuel stabilization efficiency. Foams 2, 3 and 4 were reticulated (opencell) types with high permeability to gases (air flow >30 m³ h⁻¹) andtherefore, presumably, to liquids. Each of these foam samples gave atleast a 75% reduction in total insolubles, compared to the referencefuel. Foams 1, 5, 6 and 7 were closed cell PU foams. Total insolublesreductions for foams 1, 6 and 7 were comparable to those of thereticulated foams; however, foam #5 (FIG. 3), which had a much lower airflow (0.51 m³ h⁻¹), gave a relatively poor reduction (55%) in totalinsolubles. The low permeability of this foam may have reduced freeaccess of fuel to the interior of the sample during ageing, loweringoverall fuel/foam contact, thus affecting its fuel stabilizinginfluence. It may be noted that difficulty was experienced in removingresidual fuel from this sample, even after hexane rinsing, and heating(115° C.) in a vacuum oven. This also was attributed to the very lowpermeability of the foam.

Ageing of the fuel blend in the presence of foams #1 and #2 (FIG. 2)clearly had a beneficial effect on aged fuel filterability. FiltrationIndex values for the 80° C.--13 day fuel stress data for the foams arein the "good" to "fair" categories, while the reference fuel FiltrationIndex indicates "poor" filterability. Similar results were obtained forthe 120° C. data, even though for foam #1, the particulate measurementwas boosted by foam fragments from thermal decomposition of the foam.

Filtration Index values shown in FIG. 3, however, indicate "poor"filterability for all fuel samples, whether aged with or without addedPU foams. Particulate levels were much lower for fuels aged in thepresence of foams, but their filter blocking tendencies appear to beworse than for the reference fuel. The two sets of filtration data inFIGS. 3 and 4 cannot be compared directly, since different fuelstressing conditions were used. It has been observed previously thatfiltration characteristics of fuels can be worse after shorter term thanlonger term ageing at the same temperature. This may be related tochanges in particle sizes of fuel deposits with time.

The performance of PU foams 1 and 2 for fuel stabilization was comparedwith that of fuel soluble additives FOA-3 and FOA-15 (FIG. 2). In aprevious study, using a similar fuel blend, reductions in totalinsolubles in the range 50-70% were achieved when FOA-3 was added at aconcentration of 24 ppm. At the same concentration, this additiveeffected reductions in total insolubles of 55% for the fuel blend usedin the present study, a result significantly lower than that achievedwith the immersed PU foams (>80% reduction). The Filtration Index wasalso quite high in comparison.

The FOA-15 additive, however, gave total insolubles reduction comparableto those effected by the PU foams, although the Filtration Index of 1.9was significantly higher. This may have been due to the presence of thedispersant component in FOA-15, which has been demonstrated previouslyto adversely affect aged fuel filterability. It was concluded from thisstudy that the PU foams were at least as effective as these fuel-solubleadditives in suppressing distillate fuel degradation during ageing.

The effectiveness of PU foam as a fuel stabilization additive wasexamined using fuel blend doped with known deposit promoters thiophenol,chloroacetic acid and copper naphthenate.

Addition of thiophenol to the fuel blend, at concentrations of 0.001Mand 0.003M, caused increases in total insolubles from 43 mg/l, for theundoped fuel, to 217 and 600 mg/l, respectively, during fuel ageing at65° C. for 43 days (FIG. 4). Very large increases in adherent gum levelswere observed; the very low relative levels of particulate matter (2mg/l, or less) were consistent with Filtration Index values whichindicated "good" filterability. Thus, filterability evaluation in thiscase was not an indication of actual fuel stability.

Dramatic reductions in total insolubles were achieved when PU foamsamples were immersed in the solutions of thiophenol in the fuel (FIG.4). The magnitude of insolubles reduction (>98%) indicated that thepowerful deposit promoting activity of thiophenol had been completelycounteracted by the PU foam additive.

Filtration Index values of the fuel aged with the thiophenol/foamcombination rated "good" in the filterability classification.

It may be seen from FIG. 4 that similar results were obtained usingsolutions of chloroacetic acid (0.001M and 0.003M) in the fuel blend.Respective increases in total insolubles of 2.4 and 3.6 fold at the twoacid concentrations were not as large as those for thiophenol solutions(5.0 and 14.0 fold). Immersed PU foam additives effectively counteractedthe destabilizing effect of the chloroacetic acid, although not quite tothe same degree observed for the thiophenol solutions. The acid had amuch greater tendency than the thiol to increase the proportion ofparticulate matter, relative to adherent gum, and this was reflected inthe "poor" filterability rating for fuel aged with acid alone. Incombination with PU foam additive, particulate was reduced by >80% forthe acid/fuel solutions, and improved Filtration Index values wereobserved (2.6:1.1 for 0.001M solution, 2.6:1.8 for 0.003M solution).

Addition of copper (as copper naphthenate) to the reference blend at aconcentration of 0.25 mg/l caused an increase in total insolubles from39 mg/l (with no additive) to 70 mg/l, during ageing at 80° C. for 13days (FIG. 5). The catalytic effect of copper in promoting depositformation was not enhanced when the copper concentration was increasedto 1.00 mg/l. The ratio of particulate to adherent gum which formed inthe presence of copper was about 4:1, compared to about 2:1 forchloroacetic acid, again much greater than that for thiophenol.Filtration Index values for the fuel aged with copper present did notincrease, relative to the aged reference fuel.

Ageing of the 0.25 mg/l solution of copper in fuel with PU foam additivedecreased total insolubles formation by 89%. The actual amount of totalinsolubles formed (8 mg/l) was the same as when the reference fuel (withno copper added ) was aged with PU foam, indicating that the depositforming action of the copper had been fully counteracted by the foam.With the 1.00 mg/l copper solution, the foam was slightly lesseffective, giving an 81% reduction in insolubles. However, the excellentperformance of PU foam as a copper deactivator is evident. Significantimprovements in Filtration Index values were also achieved when thecopper solutions were aged with PU foam samples (FIG. 5).

Polyolefin solids which are capable of acting as preferred depositionsites for molecules of insoluble material, and which possess an affinityfor precursors of insoluble material in fuel, showed fuel stabilizationproperties. The active sites arise from heteroatom functional groups orthe addition of polar additives such as Ciba-Geigy Tinuvin 770, Tinuvin622 and Chimassorb 944.

Knitted high density polyethylene cloth, impregnated with 0.45 weightpercent of polymeric hindered amine light stabilizers containing2,2,6,6-tetramethylpiperidine moieties (Tinuvin 622 and Chimassorb 944)were immersed for 12 days at 80° C. in a fuel blend similar to thatdescribed above. Total insolubles were 11 mg/l, compared to 58 mg/l forthe fuel aged with no additive, a reduction of 81%. Filtration Indexvalues for these aged fuel samples were 1.6 and 4.5, respectively.Polypropylene, and high and low density polyethylene without the polarcopolymer additives were ineffective as fuel stabilizers, giving noreduction in total insolubles compared to the reference fuel in theabove experiments.

Thus, the use of polyolefin solids, in conjunction with polarcopolymers, has been successfully demonstrated for fuel stabilization.

Woven Kevlar [poly(1,4-phenylene terephthalamide)] cloth, nylon 6--6cord and polyester fibres showed fuel stabilization properties whenimmersed in the test fuel (14 days, 80° C.) containing 0.001 mol/lchloroacetic acid (deposit promotor). The reference fuel withoutadditive produced 97 mg/l of total insolubles whereas that produced infuel aged in the presence of the above materials were 45, 50 and 8 mg/l,respectively.

Synthetic and natural fibrous materials were found to be fuelstabilizers with respect to fuel colour, before and after fuel ageingfor 7 days at 80° C. It can be seen from Table 3 that the fibrousmaterials reduced fuel colour readings by 0.5-1.5, compared to the agedreference fuel with no additive. Under controlled laboratory conditionsof fuel ageing, colour reductions of this order in comparative tests,using the same fuel blend, indicate significant reductions in theamounts of insoluble fuel degradation products. Evaluation of thesesolid additives by this means has illustrated their effectiveness asfuel stabilizing additives.

                  TABLE 3                                                         ______________________________________                                                 Fibre   Polar Chain     Aged fuel                                    Trade Name                                                                             wt.     Functional      Colour (ASTM                                 of Fibre (g/l)   Groups          D1500).sup.a                                 ______________________________________                                        DACRON 45                                                                              4.5     ESTER           3.0                                          ORLON 75 2.0     ACRYLONITRILE   3.0                                          NYLON 6-6                                                                              1.6     AMIDE           2.0                                          COTTON   1.9     HYDROXYL; ETHER 2.5                                          WOOL     1.7     AMIDE; CYSTINE  2.0                                          RAYON    3.8     ACETATE         3.0                                          (BLANK   --        --            3.5                                          FUEL)                                                                         ______________________________________                                         .sup.a Initial fuel colour: 1.5                                               Fuel Stress: 80° C., 7 days                                       

This invention is applicable to all distillate fuel tanks, includingstatic storages, vehicle fuel tanks and aircraft fuel tanks.

Adoption of the present invention provides the following advantages:

(i) Insoluble solid additives possessing the properties described, wheninserted in fuel systems provide a continuously efficient, passiveenvironment for minimising the effects of fuel instability duringstorage.

(ii) Such solids have the capability of maximising storage stability ofvery unstable fuels containing cracked refinery stock. This issignificant, since refineries world-wide are increasing the proportionof cracked stock into middle distillates.

(iii) Less demand would be placed upon expensive refinery hydrotreating,or the use of fuel stability additives, for fuel systems whichincorporated suitable solid additives.

(iv) Solids such as polyurethane foams, polyester fibre and Kevlar clothare stable and non-toxic to handle. No precautions are required in theiruse. This may be contrasted with chemical additives, which are generallytoxic and require protective equipment to be handled in theirconcentrated form.

It is envisaged that the fuel stabilizing system of this invention willfind the following application:

(a) For insertion in Defence fuel systems and strategic installationswhere there is a requirement to protect fuel from chemical degradation(most Defence materiel has this requirement).

(b) For use in vehicle fuel tanks in which the deterioration ofdistillate fuel may occur due to chemical ageing. This includescommercial diesel powered vehicles, diesel powered rural equipment andmarine and air craft.

(c) For use in storage tanks of all sizes in which distillate fuel iskept for any length of time, when degradation products are likely toincrease in concentration and cause malfunction in equipment whensubsequently used.

The claims defining the invention are as follows: We claim:
 1. A methodfor minimizing the chemical degradation of a liquid hydrocarbondistillate fuel during storage for an extended period of time,saidliquid hydrocarbon distillate fuel containing cracked products derivedfrom a heavy crude or distillation fraction, said cracked productsincluding chemically unstable species that promote fuel degradation uponstorage for an extended period of time leading to a build-up ofinsoluble particulate material, comprising the step of storing saidhydrocarbon fuel in contact with a polymeric solid which is capable ofremoving or counteracting fuel components catalyzing or participating indegradation reactions and which is selected from the group consisting ofpolyurethane foam, polyolefin fiber, polyacrylonitrile, cotton, wool,and polyacetate, whereby the amount of said chemically unstable speciesin said liquid hydrocarbon fuel is reduced.
 2. The method according toclaim 1, wherein the polymeric solid is in the form of cellular foam,sponge, mesh, woven fabric, naturally entwined or bound fiber bundles,surface coatings, strips, films, or solids encapsulated in afuel-permeable container.
 3. The method according to claim 1, whereinthe polymeric solid is an open-cell polyurethane foam.
 4. The methodaccording to claim 1, wherein the polymeric solid is a polyether-basedpolyurethane foam.
 5. The method according to claim 1, wherein thepolymeric solid is a polyester-based polyurethane foam and the liquidhydrocarbon fuel contains aromatic hydrocarbons.
 6. The method accordingto claim 1, wherein the polymeric solid is a polyurethane foam based onpoly(diethylene glycol) adipate or poly(oxypropyl) poly(oxyethyl)glycerol or on an acrylonitrile-styrene modified polyoxyalkylenepolyether resin.
 7. The method according to claim 1, wherein thepolymeric solid is a polyethylene cloth containing heteroatom functionalgroups or polar additives.
 8. The method according to claim 1, whereinthe polymeric solid is poly(1,4-phenylene terephthalamide) cloth.
 9. Themethod according to claim 1, wherein the polymeric solid is apolyurethane foam and the method of storing the hydrocarbon fuel incontact with the polymeric solid is by adding the polymeric solid to astorage tank containing the fuel in an amount of 1 to 5 grams ofpolyurethane foam per liter of fuel.
 10. A method for minimizing thechemical degradation of a liquid hydrocarbon distillate fuel duringstorage for an extended period of time,said liquid hydrocarbondistillate fuel containing cracked products derived from a heavy crudeor distillation fraction, said cracked products including chemicallyunstable species that promote fuel degradation upon storage for anextended period of time leading to a build-up of insoluble particulatematerial, comprising the step of contacting said hydrocarbon fuel with apolymeric solid which is capable of removing or counteracting fuelcomponents catalyzing or participating in degradation reactions andwhich is selected from the group consisting of polyurethane foam,polyolefin incorporating a polar copolymer, polyamide, Nylon 6--6,polyester fiber, polyacrylonitrile, cotton, wool, and polyacetate,whereby the amount of said chemically unstable species in said liquidhydrocarbon fuel is reduced.